1. Field of the Invention
The present invention relates to coating compositions containing a mixture of at least two ethylenically unsaturated polyurethanes as binders.
2. Description of the Prior Art
UV curable coatings are one of the fastest growing sectors in the coatings industry. In recent years, UV technology has made inroads into a number of market segments like fiber optics, optical- and pressure-sensitive adhesives, automotive applications like UV cured topcoats, and UV curable powder coatings. The driving force of this development is mostly the quest for an increase in productivity of the coating and curing process. In automotive refinish applications where minor repairs need to be performed swiftly and at ambient temperature, UV technology promises to significantly increase the throughput of cars in a body shop. The development of refinish applications breaks new ground in UV technology. Safety concerns associated with the use of UV lamps in body shops as well as economic constraints will likely preclude the use of high intensity light sources. Relatively inexpensive low intensity lamps that emit only in the UV-A region of the electromagnetic spectrum are taking their place, thus posing new challenges to resin developers and formulators.
UV curable coating compositions are known in the art. See, e.g., U.S. Pat. Nos. 4,188,472; 4,340,497; 6,465,539; and 6,534,128; and published PCT applications WO 97/04881 and WO 03/027162. In addition, U.S. Pat. No. 5,777,024 is directed to the reaction product of a low viscosity isocyanurate group-containing polyisocyanate, which has been modified with allophanate groups to reduce its viscosity, with hydroxyl-functional ethylenically unsaturated compounds such as hydroxyalkyl (meth)acrylates and lactone modified versions of these (meth)acrylates. U.S. Pat. No. 6,306,504 is directed to a coating composition for polycarbonate which contains the reaction product of a low viscosity polyisocyanate, such as a low viscosity isocyanurate group-containing polyisocyanate, with a low molecular weight, hydroxyalkyl acrylate. Japanese Kokai 2004-338214 is directed to a coating composition containing a mixture of the reaction product of an isocyanurate group-containing polyisocyanate with 3-acryloyloxy-2-hydroxypropyl methacrylate and a copolymerizable monomer. Japanese Kokai 2004-059820 is directed to a photocurable composition prepared by reacting a 2-hydroxyethyl methacrylate-isobornyl methacrylate-methylmethacrylate copolymer with the reaction product of isophorone diisocyanate and 3-acryloyloxy-2-hydroxypropyl methacrylate and mixing the resulting product with a urethane acrylate oligomer. Finally, Japanese Kokai 2003-342525 is directed to a coating composition containing the reaction product of an isocyanurate group-containing polyisocyanate with 2-hydroxy-3-allyloxypropyl methacrylate.
Also known are compositions that are curable using UV radiation having a very low UV-B content and substantially no UV-C content (see, e.g., U.S. Patent Application Publication 2003/0059555 and U.S. Pat. No. 6,538,044). In order to be commercially useful as a clear coat, particularly for the refinish market, a coating composition must be curable in a relatively short time, preferably under relatively low intensity radiation. In addition, such coatings must exhibit a balance between hardness (as measured by pendulum hardness) and flexibility (as measured by impact resistance). None of the coating compositions described in the above literature meet these criteria.
Copending application, U.S. Ser. No. 10/754,244, filed Jan. 9, 2004, is directed to UV curable coating compositions containing a mixture of two urethane acrylates. One of the urethane acrylates is prepared by reacting a polyisocyanate with a lactone-acrylate adduct and the other is prepared by reacting a polyisocyanate with a hydroxyalkyl (meth)acrylate. While coatings prepared from these urethane acrylate mixtures possess a combination of hardness and flexibility, to obtain the hardnesses required for certain applications, such as automotive refinish applications, it is necessary to add di-, tri-, and tetra-acrylates to the coating compositions as reactive diluents.
Accordingly, it is an object of the present invention to reduce or eliminate the need for reactive diluents and still achieve the combination of hardness and flexibility that is required for certain applications, such as automotive refinish applications. This object may be achieved with the compositions of the present invention which are described in more detail hereinafter.